Why does a student mention the Mona Lisa?

Listen to part of a lecture in an art history class.
P: Today I’m going to introduce a small oil painting by the 17th century Dutch artist Johannes Vermeer— ’Girl with a Pearl Earring.’ It’s signed by the artist, but not dated. This piece is probably the most often reproduced of all of Vermeer’s work. At first, it looks like a portrait, but technically it’s not. It belongs to a genre known in Dutch as a "tronie." Tronie is a category that’s familiar to scholars for many examples by other 17th century Dutch masters. The subjects of tronies were not intended to be recognized as individuals, but as interesting characters or archetypes. Typical characters in this genre include the dashing soldier and the wise teacher. Tronies were usually depicted with just a head and shoulders, whereas portraits were normally half-length or full-length, and they were much more formally composed. As you can see, the girl’s wearing a yellow garment and a blue and yellow turban. Her earrings are a stunning teardrop-shaped piece of glass that has been varnished to look like a gigantic pearl. The subject is turning to look at us over her left shoulder.
S1: It’s almost like a snapshot.
P: Well, you’re right, John. It is not a formal photo. It does seem spontaneous, but there’s actually painstaking craft on display here. Can you see the way the highlight on the front of the turban lights her head? The light also bounces from her collar, and the effect of this is softly lighting the jaw from below. This lighting scheme’s echoed in the earring. Her lips are slightly open, making it possible for the artist to create the most remarkable detail of all. On the left, a narrow stream of light passes between her lips and strikes the opposite corner of her mouth as a little point of light and then stands out and spreads across her cheek. This shows Vermeer’s powerful observation of optical effects and his amazing ability to translate that observation into paint. Another point I want to make is how allusive the girl is. Would anyone like to have a guess why?
S1: Well, maybe because there aren’t any walls or furniture telling us where the girl is or her social status or when she lived.
P: Exactly. No setting, no props, no specifics in her costume. Not even a hairstyle. So, she doesn’t belong to any particular time or place. I think this allusive, this enigmatic quality is responsible in part for the paintings popularity.
S2: It reminds me of what people say about the Mona Lisa.
P: Right! The great Italian Renaissance painter Leonardo da Vinci also created a sense of mystery with Mona Lisa’s hard-to-interpret smile. You know, a few years ago a computer analysis was done on Mona Lisa’s face using software that evaluates facial expressions. According to its interpretation, it found that she was happy, disgusted, fearful, and angry all at once. Anyway, getting back to our topic, It’s no coincidence that ’Girl with a Pearl Earring’ has captured the imagination of contemporary writers. Have any of you read the best-selling novel called ’Girl with a Pearl Earring?’ Or seen the movie adaptation of it? There’s also an opera, partially based on this painting. Well, all these contemporary works fill in some of the gaps by suggesting missing facts in our understanding of this image. But despite all this, the painting’s allusive quality, the girl’s features are quite specific, so it’s unlikely that she’s imaginary.
S2: I heard she was Vermeer’s oldest child.
P: Yes, an interesting theory. Vermeer and his wife had many children. The eldest, Maria, was probably born in 1654 and ’Girl with a Pearl Earring’ bears a striking resemblance to a girl in another Vermeer painting, also undated.
The two leading experts on Vermeer are close to agreeing that ’Pearl Earring’ was painted when Maria would have been between 10 and 12. Those same scholars believe the other painting was made a year later, but keep in mind that these dates are based solely on the expert’s opinions of where these paintings fit into the development of Vermeer’s style. In my view, the style of these paintings could fit just as well into when Maria was a couple of years older.
S1: But, how does any of this prove that the girl pictured was Maria?
P: After Vermeer’s death, his wife had to sell his artwork to pay bills, but she tried to keep the other painting I just mentioned in her family. I suspect it was because it included an image of one of her children.